1. Field of the Invention
The present invention relates to a magnetic field generating apparatus. More particularly, it concerns a magnetic field generating apparatus of a permanent magnet type suitable to obtain an image of any of desired sections of a subject on the basis of the nuclear spin density information or relaxation time information thereof provided with use of nuclear magnetic resonance imaging (MRI) technology.
2. Description of the Prior Art
In order to generate the NMR phenomenon, required is a static magnetic field of spatially uniform strength in an unchanging direction with time. In order to measure in a spatially wide region, such as a human body, for example, requires a magnetic field generating apparatus that can generate the static magnetic field of 0.04 to 2 T (tesla equal to 10,000 gauss) with the uniformity below some 10 ppm in the measuring spherical region of 30 to 50 cm diameter. There are three types of magnetic field generating apparatus: normal conduction magnet, superconduction magnet, and permanent magnet types.
A previous magnetic field generating apparatus of permanent magnet type for the MRI apparatus has a hole-like space or a hollow space of a polygonal sectional shape surrounded by a plurality of permanent magnets as described in the Japanese Patent Application Laid-Open 62-177903. The central area of the hollow space has the object put therein and is made to generate the uniform static magnetic field.
The previous magnetic field generating apparatus is described below by reference to FIGS. 11 to 13. In FIG. 11, a yoke 1 is a member forming a magnetic circuit together with a permanent magnet 2, which is described below. The yoke 1 is formed of a soft magnetic material, such an iron plate or silicon steel plate, to a square cylinder shape. The yoke 1 has a pair of permanent magnet elements 21 and 22 arranged on an upper and lower inside walls thereof facing each other as shown in FIG. 12. The permanent magnet elements 21 and 22 serve for generating a main magnetic flux in a vertical direction to the inside walls as shown, each being formed, for example, like a plate having a trapezium section. The yoke 1 also has two pairs of permanent magnet elements 23 and 24 and 25 and 26 arranged on a right and left inside walls thereof facing each other as shown in the figures. These permanent magnet elements 23 to 26 serve for compensating for a uniformity of a magnetic field by the permanent magnet elements 21 and 22, each being formed, for example, like a plate having a scalene triangle section. Each of the permanent magnet elements 23 to 26 is stuck, for example, on its short-side surface to a corresponding slant of the permanent magnet elements 21 and 22. Thus, the plurality of permanent magnet elements 21 to 26 form a permanent magnet 2 having a polygonal section. The permanent magnet 2 forms a through-hole or hollow space 5 in which an object can enter and be surrounded by the permanent magnet elements. It should be noted that the permanent magnet elements 23 to 26 are symmetrically arranged vertically and horizontally around a center axis 6 of the permanent magnet 2. It should also be noted that each of the permanent magnet elements 23 to 26 is uniformly magnetized in a direction perpendicular to their respective boundary surfaces with the through-hollow space 5 as indicated by the respective arrows in FIG. 11. With the construction described above, a uniform static magnetic field 8 is generated, for example, vertically from bottom to top in a measuring region 7 of a central area of the through-hollow space 5.
With the construction of the previous magnetic field generating apparatus described above, as shown in FIG. 12, the magnetizations of the upper and lower magnet elements 21 and 22 are directed right upward as looked into away from an X-Y section. The magnetizations of the upper right and left side magnet elements 23 and 24 are obliquely directed outward. The magnetizations of the lower right and left side magnet elements 25 and 26 are obliquely directed inward. The magnetizations of the permanent magnet elements 21 to 26, as shown in FIG. 12, are all vertically directed perpendicular to the center axis 6 as looked into away from a Y-Z section. That is, the magnet elements 21 to 26 are all magnetized in the same direction at right angles to the center axis 6.
However, as the magnetic field generating apparatus for the MRI apparatus has to have the subject put in the through-hollow space 5 in the central area thereof, as shown in FIG. 13, the magnetic field generating apparatus has an opening 9 on each of both longitudinal ends of the permanent magnet 2 formed of the permanent magnet elements 21 to 26. For this reason, the magnetic field generated by the permanent magnet elements 21 to 26 extends out of the openings 9. This results in that the uniformity of the static magnetic field 8 is made worse in the measuring region 7 in the central area. Thus, the quality of the section image obtained is deteriorated. In order to prevent such a bad typical, it is effect to maker longer the depth-wise length of the permanent magnet elements 21 to 26 in the direction Z in FIG. 13. However, this measure involves an economic problem in that an amount of expensive permanent magnet material must be increased.
On the other hand, another type of previous magnetic field generating apparatus is proposed in the Japanese Patent Application Laid-Open 61-276308. This previous apparatus has some portions cut out of each of permanent magnets forming it. The portions are arranged at proper intervals in a longitudinal direction of the apparatus so that the uniform magnetic field can be obtained. However, there are gaps between the portions of the permanent magnets in the longitudinal direction. The gaps leak out the magnetic flux. This results in the problem in that the uniformity of the magnetic field strength is made worse in the measuring region in the central area.
Still another type of previous magnetic field generating apparatus is proposed in the Japanese Patent Application Laid-Open 2-83903. The apparatus, as shown in FIG. 14, has no yokes on the periphery, but a ring-like permanent magnet element group 10 arranged on the central portion and ring-like permanent magnet element groups 11 and 12 arranged on both sides. The permanent magnet element groups 11 and 12 arranged on both sides is made thicker than the permanent magnet element group 10 arranged on the central portion to improve the uniformity of the magnetic field. In this previous apparatus like the one in FIG. 13, the magnetizations of the permanent magnet element groups 10, 11, and 12 are all vertically directed perpendicular to a center axis 6'. However, the apparatus involves an economic problem that the amount of expensive permanent magnet material must be increased to obtain the fully uniform magnetic field in the measuring region in the central area inside the hollow area.